The present disclosure generally relates to coating methods and compositions for turbine components. These coatings and processes are especially suitable for hydroelectric turbine components, which exhibit improved silt erosion resistance from the coating.
Components are used in a wide variety of industrial applications under a diverse set of operating conditions. In many cases, the components are provided with coatings that impart various characteristics, such as corrosion resistance, heat resistance, oxidation resistance, wear resistance, erosion resistance, and the like.
Erosion-resistant coatings are frequently used on hydroelectric turbine components, and in particular, the runner and the guide vanes, for Francis-type turbines, and the runners, needles, and seats for Pelton-type turbines, as well as various other components that are prone to silt erosion. Erosion of these components generally occurs by impingement of silt (sand in the water) and particles contained therein (e.g., SiO2, Al2O3, Fe2O3, MgO, CaO, clays, volcanic ash, and the like) that are carried by moving bodies of water. Existing base materials for hydroelectric turbine components such as martensitic stainless steels do not have adequate erosion resistance under these conditions. For example, hydroelectric turbine components when exposed to silt in the rivers that exceed 1 kg of silt per cubic meter of water have been found to undergo significant erosion. This problem can be particularly severe in Asia and South America where the silt content during the rainy season can exceed 50 kg of silt per cubic meter of water. The severe erosion that results damages the turbine components causing frequent maintenance related shutdowns, loss of operating efficiencies, and the need to replace various components on a regular basis.
In order to avoid erosion problems, some power stations are configured to shut down when the silt content reaches a predetermined level to prevent further erosion. Oftentimes, the predetermined level of silt is set at 5 kg of silt per cubic meter of water. In addition to shutting down the power stations, various anti-erosion coatings have been developed to mitigate erosion. Such coatings include ceramic coatings of alumina, titania, chromia, and the like; alloys of refractory metals, e.g., WC-CoCr coatings; WC-Co, WC-CoCr+NiCrBSi coatings; carbides; nitrides; borides; or elastomeric coatings. However, current compositions of the above noted materials and processes used to apply them generally yield coatings that are not totally effective during prolonged exposure to silt.
Current erosion resistant coatings are usually applied by thermal spray techniques, such as air plasma spray (APS), and high velocity oxy-fuel (HVOF). One limitation to current thermal spray processes is the limited coating thicknesses available due to high residual stress that results as thickness is increased by these methods. As a result, the final coating is relatively thin and fails to provide prolonged protection of the turbine component. Other limitations of these thermal spray processes are the oxidation and decomposition of the powder feed or wire feed stock during the coating process that form the anti-erosion coating, which can affect the overall quality of the finished coating. For example, present thermal spray processes such as plasma spray, wire spray, and HVOF are currently used for coating turbine components. These thermal spray processes generally leave the resulting coating with relatively high porosity, high oxide levels, and/or tends to decarborize primary carbides, if present in the coating. All of these factors have significant deleterious effects at reducing erosion resistance of the coatings.
Of all the different prior art deposition processes, HVOF yields the most dense erosion resistant coatings and as such, is generally preferred for forming erosion resistant coatings. However, even HVOF yields coatings with high residual stress, which limits the coating thickness to about 500 microns (0.020 inches) in thickness. Also, because of the gas constituents used in the HVOF process and resulting particle temperature and velocity, the so-formed coatings generally contain high degrees of decarburization, which significantly reduces the coating erosion resistance.
Preparation of erosion resistant coatings must also account for fatigue effects that can occur in the coating. The fatigue effects of a coating have often been related to the strain-to-fracture (STF) of the coating, i.e., the extent to which a coating can be stretched without cracking. STF has, in part, been related to the residual stress in a coating. Residual tensile stresses reduce the added external tensile stress that must be imposed on the coating to crack it, while residual compressive stresses increase the added tensile stress that must be imposed on the coating to crack it. Typically, the higher the STF of the coating, the less of a negative effect the coating will have on the fatigue characteristics of the substrate. This is true because a crack in a well-bonded coating may propagate into the substrate, initiating a fatigue-related crack and ultimately cause a fatigue failure. Unfortunately, most thermal spray coatings have very limited STF, even if the coatings are made from pure metals, which would normally be expected to be very ductile and subject to plastic deformation rather than prone to cracking. Moreover, it is noted that thermal spray coatings produced with low or moderate particle velocities during deposition typically have a residual tensile stress that can lead to cracking or spalling of the coating if the thickness becomes excessive. Residual tensile stresses also usually lead to a reduction in the fatigue properties of the coated component by reducing the STF of the coating. Some coatings made with high particle velocities can have moderate to highly compressive residual stresses. This is especially true of tungsten carbide based coatings. Although high compressive stresses can beneficially affect the fatigue characteristics of the coated component, high compressive stresses can, however, lead to chipping of the coating when trying to coat sharp edges or similar geometric shapes.
Accordingly, there remains a need in the art for improved coating methods and coating compositions that provide effective protection against erosion resistance, such as is required for hydroelectric turbine components. Improved coating methods and/or coating compositions on regions of hydroelectric turbine components desirably need coatings with a combination of high erosion resistance, low residual stresses, and higher thickness to provide a coating with long life and high erosion resistance in high silt concentration operating conditions.